Voltage sources whose source voltage and internal resistance can be preset independently of each other, i.e., can be adjusted, are needed for testing electronic equipment or circuit groups, for example, to determine their reaction to different input switching modes. The voltage source connected to the inputs is then set at different source voltages and internal resistance values in succession. A circuit arrangement which makes this possible contains an adjustable voltage source with which a resistance arrangement that can be switched between several resistance values is connected in series and serves as a switchable internal resistance of the voltage source. The switching device can be a manually operated selector switch but it is also possible to implement such switching devices in the form of a relay arrangement.
When such an arrangement consisting of a voltage source and variable internal resistance is to be switched, not manually but instead by electric selection signals, optionally even automatically through a predetermined sequence of source voltage values and internal resistance values, this can be accomplished with a digitally adjustable voltage source in combination with a relay circuit. Use of electronic analog switches for switching the internal resistance values is impossible in many applications because such switch elements do not have sufficient dielectric strength and their inherent resistance can cause measurement errors in the range of low internal resistance values.
Therefore, with voltage sources of the type described here, a mechanical switch contact is needed for each preset internal resistance value. In the sense of achieving the greatest possible operating reliability, the switchable internal resistors must also be able to withstand the high loads that occur in the event of a short circuit. This requires high expenditures in terms of space and costs.
The purpose of this invention is to implement the independent adjustment of source voltage and internal resistance of a voltage source in such a way that no mechanical switch contacts and no separate high rated resistors are necessary.
This invention solves this problem for a circuit arrangement of the type described initially by means of a computing circuit for calculating a reference parameter for a current or voltage regulator that forms the output of the voltage source from a measured parameter that corresponds to the output voltage or the output current and the input parameters corresponding to the preset values.
A circuit arrangement according to this invention thus does not contain the series circuit of a voltage source with a resistance arrangement but instead the current or voltage regulator serves to simulate the behavior of a voltage source with preset values for source voltage and the internal resistance at its output terminals by calculating its reference parameter according to the response that is to be simulated. In this way the switchable internal resistors as well as the switch device with the mechanical switch contacts become superfluous and the reference parameters to be supplied to the circuit arrangement in accordance with the values to be preset can be set with analog switches because they only have a controlling function.
The calculation of the reference parameter for the current or voltage regulator to be performed with the computing circuit is very simple because it is based on the fact that the behavior of a voltage source at a preset source voltage and internal resistance value can be described completely by the output voltage and the output current. The output voltage and the output current of a voltage source can be represented by forming a simple difference and product depending on the internal resistance and source voltage. Therefore, the circuit arrangement according to this invention is further refined in that the computing circuit contains an adder and a multiplier in series connection, each of which receives one of the two input parameters. The difference formed from the output voltage and source voltage which is necessary to simulate the response of the voltage source can be determined very simply with a known amplifier due to the fact that a control voltage that is proportional to the output voltage and a control voltage that is proportional to the source voltage and is of opposite sign are supplied to its inputs. Then the summation amplifier has the advantage in comparison with a digital adding circuit that its amplification can be adjusted, e.g., in a feedback path, so the difference between the output voltage and the source voltage can be varied in this way easily by a factor that is proportional to the internal resistance in accordance with the response of the voltage source in simulating the output voltage, and is inversely proportional to the internal resistance in simulating the output current. This yields a very simple circuit where only a single amplifier is provided in the computing circuit so the source voltage and the internal resistance can be adjusted on this amplifier and the difference can be formed and the multiplication can be performed for calculating the reference parameter for the downstream current and/or voltage regulator simultaneously.
Thus an advantageous version of this invention consists of the fact that the summation amplifier has a feedback path that can be adjusted according to different preset values of the internal resistance. In such a feedback path, a normal multiplying digital-analog converter can be provided as an impedance network to adjust different internal resistance values. Converters of this type are known to need a virtual mass point for current summation. Such a mass point is also provided with summation amplifiers. The advantage of using such a multiplying converter consists of the fact that it permits multistage adjustment of the internal resistance on the summation amplifier with commercial integrated circuits.
If when using a current regulator the measured parameter corresponding to the output voltage is measured with a voltage sequence circuit, then especially in the case of a high internal resistance value or small loads connected to the circuit, their high input resistance achieves the effect that the output current of the circuit corresponds pratically to the output current of the current regulator supplying the output current, because the input current of the voltage measurement circuit is then negligibly small.
A circuit according to this invention, especially in the version with a summation amplifier, is especially suitable for setting complex internal resistance values because the impedance network in the feedback path of the summation amplifier must then be formed only inductively or capacitively accordingly.